The effect of hydrogen donors in thermal cracking (non-catalytic cracking) of heavy stocks is well understood and various sources of hydrogen donors have been described. In U.S. Pat. No. 3,238,118, the hydrogen donor for thermal cracking of crude still bottoms is the fraction of hydrocracker product boiling above the naphtha range, that is, higher boiling than 430.degree. F. That fraction will contain the polycyclic aromatics and hydrogenated polycyclics generated during hydrocracking including naphthalene, tetralin and higher together with other compounds of like boiling range and including compounds having functional groups to the extent these survive the conditions in the hydrocracker.
A later U.S. Pat. No. 4,090,947 describes hydro-treating of heavy gas oils, including heavy gas oil from a premium coker, to generate a hydrogen donor diluent stream which is then blended with fresh charge for thermal cracking. Such prior practices involve a catalytic hydrogenation of a stream which contains all the components normally present in the fraction to be hydrogenated; including nitrogen, metal and sulfur bearing compounds and compounds such as asphaltenes which have a high propensity for formation of coke. The detrimental effect of such components on hydrogenation catalysts is well known in the art of petroleum processing.